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The vertical excitation and emission energies of the cytosine oxo-amino form were calculated at the TD DFT level of theory with several functionals, basis sets, and solvents with different polarity (PCM). They were compared with the experimental UV absorption and fluorescence maxima, revealing that the minimal deviation of the vertical excitation energies from the of UV absorption maxima can be achieved when the hybrid functional B3LYP is applied within the calculations. Regular correlations like curves with saturation between the vertical excitation/emission energies and the dielectric constants were registered. The relaxation of the 1 ππ * excited state through an ethylene-like conical intersection S 0 /S 1 should occur through decrease of the decay rate (commented qualitatively here) with the rise of the solvent polarity., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)